How do you get rid of graffiti? Chemistry, of course. Science 2.0 says that a simple dose of 95 percent ethanol does a surprisingly good job at wiping away spray paint. But it’s a little more complicated than just dumping alcohol on everything:
That’s one of the challenges in removing graffiti. The most common medium, spray can paint, can host a variety of compounds: polyurethanes, lacquers and enamels. For each of these, there are solvents capable of forming intermolecular bonds with the compounds that are stronger than those between the latter and the background. Examples include butanone (MEK= methyl ethyl ketone) and xylene. But in attempting to remove graffiti, there’s the risk of letting the paint penetrate deeper and of damaging the surface itself. It’s advisable to begin by testing a solvent on small areas.
After you apply whatever remover, there might still be traces of the pesky paint leftover. Don’t worry, chemistry can solve that too:
If there are still residual pigments after treatment with a solvent, they can be bleached with swimming pool disinfectant: calcium hypochlorite, Ca(OCl)2. Since this compound is only slightly alkaline (it’s the product of a weak acid and strong base), it’s innocuous towards both acid-sensitive and alkali-sensitive surfaces. Most commercial products use a shotgun approach by blending several agents. For example, an old recipe uses a blend of Ca(OCl)2, pine oil and ammonia. Another employs base, ether, ethanol and a ketone.
When you don’t care that much about your building or surface, you can just go for it. But on historic landmarks, or important old buildings, preservation can be tough. The U.S. National Park Service has a whole guidebook on getting rid of paint from historic masonry. Pesky kids.
Of course, graffiti is nothing new. The Romans scratched on their walls, even Stonehenge has signs of ancient vandalism. In the United States, graffiti has a long cultural history too. Here’s PBS on how spray paint made its way from subway cars to galleries:
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